This invention relates to a method of modifying a video signal. Applications of the method include preventing unauthorized recording of video signals, including unauthorized copying of pre-recorded videotapes, testing data decoders such as closed caption decoders and inserting visible text into a video display.
In the United States, a video signal that is broadcast for reception by a standard television receiver is composed of a succession of fields, each comprising a vertical blanking interval in which the signal level remains at or below a predetermined blanking level, and an active field interval, and each field is composed of a succession of lines, each comprising a horizontal blanking interval and an active line interval. Such a video signal is referred in the following description and in the ensuing claims as a "standard video signal".
Many prior devices have been used to modify a video signal during its blanking intervals.
U.S. Pat. No. 2,268,001 issued in 1941 to R. R. Von Felgel-Farnholz describes adding pulses just after the horizontal sync pulses. The amplitude of the pulses is about 30% of the black-to-white video range.
U.S. Pat. No. 2,303,968 issued in 1942 to White describes automatic gain control ("AGC") pulses starting at blanking level and extending to slightly greater than peak video amplitude. The AGC pulses are placed after the horizontal sync pulses and are used to control an automatic gain control in order to achieve a desired signal level.
U.S. Pat. No. 3,423,520 issued in 1969 to H. P. Kelly shows the addition of pulse pairs to blanking just before sync pulses.
U.S. Pat. No. 3,466,387 issued in 1969 to E. R. Rout describes the addition of bi-directional pulses to blanking after sync pulses. The pulses extend from below sync to about halfway through the black-to-white range.
U.S. Pat. No. 2,885,470 issued in 1959 to E. H. B. Bartelink shows the addition of square waves to one line in the vertical blanking interval. The range of the added square wave is from blanking to approximately peak white and is used for testing. Square waves having frequencies of 100 Khz, 1 MHz and 4 MHz are used.
U.S. Pat. No. 2,823,252 issued in 1958 to J. E. Bridges shows the addition of ". . . a burst of signal of frequency . . . . " FIG. 3J appears to show either a sawtooth or sine wave. FIG. 30 shows that they occupy seven lines in the vertical blanking interval and extend from blanking to approximately peak white.
U.S. Pat. No. 3,824,332 issued in 1974 to Horowitz shows the addition of low amplitude sine waves on a pedestal to eight lines in the back porch of vertical blanking.
U.S. Pat. No. 2,741,661 issued in 1956 to H. DeFrance shows the addition of sine waves between sync pulses and having a range from sync tip to blanking.
U.S. Pat. No. 3,478,166 issued in 1969 to A. M. Reiter et al. shows the addition of a sawtooth wave between sync pulses and having a range from sync tip to blanking.
U.S. Pat. No. 3,538,243 issued in 1970 to W. J. Shanahan et al shows the addition of 70 KHZ, 100 KHz and 150 KHz square waves between sync pulses and having a range from sync tip to blanking.
U.S. Pat. No. 3,383,463 issued in 1968 to W. F. Goodell et al shows the addition of a "noise signal" to the back porch of a video signal. The amplitude of the noise signal is greater than that of the overall video signal, extending from below sync to above peak white.
U.S. Pat. No. 3,749,831 issued in 1973 to Simpkins shows "digital sync pulses" and "digital code pulses" inserted during vertical sync pulses with an amplitude extending from sync tip to approximately peak white.
There are many known methods of modifying a standard video signal such that it cannot be recorded on a conventional videotape recorder, or can be recorded only in severely degraded form. All of these methods have been of limited effectiveness and usually have had serious side effects. The side effects that occur in playback can generally be attributed to design choices in the television receiver, such that receivers made by some manufacturers are more susceptible to a given side effect than those made by other manufacturers. For example, one early method involved modifying vertical synchronizing pulses, included in the video signal, and this caused the display provided by certain models of television receivers to jitter vertically while those of other manufacturers were less susceptible to this problem.
Other methods of modifying a video signal involved adding high level pulses to the video signal to cause improper operation of the automatic gain control found in many video cassette recorders (VCRs). For instance U.S. Pat. No. 4,163,253 entitled METHOD OF AND APPARATUS FOR MODIFYING A VIDEO SIGNAL TO PREVENT UNAUTHORIZED RECORDING AND REPRODUCTION THEREOF issued on Jul. 31, 1979 to Morio et al describes the addition of a high level positive pulse to the "back porch" of the horizontal blanking just after an ordinary horizontal sync pulse. The back porch is used in many television receivers for black setting, and such television receivers display a dark or extinguished picture when supplied with such video.
U.S. Pat. No. 4,631,603 entitled METHOD AND APPARATUS FOR PROCESSING A VIDEO SIGNAL SO AS TO PROHIBIT THE MAKING OF ACCEPTABLE VIDEO TAPE RECORDINGS THEREOF issued on Dec. 23, 1986 to Ryan adds not only high level positive pulses but also negative, or pseudo-sync, pulses. Since the positive pulses are added to the nominally blank lines that occur during the vertical blanking interval, they appear as retrace lines on many television receivers. Additionally, the pseudo-sync pulses cause some television sets to interpret the high level positive pulses as a normal back porch and cause darkening of the displayed picture. The added pseudo-sync pulses may also disturb the television set's phase-locked loop in the horizontal sweep, thereby causing a bend at the top of the picture.